Structure and Preparation of Alkenes.
of Alkyl Halides
Reaction type: 1,2- or ß-Elimination
- When heated with strong bases, alkyl halides typically
undergo a 1,2-elimination reactions to generate alkenes.
- Typical bases are NaOH or KOH or NaOR or KOR (alkoxide)
especially NaOEt or KOtBu in the alcohol as solvent.
- Nucleophilic substitution is a potential side reaction typically leading to alcohols or ethers. (summary)
- Alkyl halide relative reactivity order : 3o > 2o > 1o
- Alkyl halide relative reactivity order : I >
Br > Cl > F (n.b. elimination of F is rarely used).
- Product distribution:
- Regioselectivity is usually controlled by the relative
stability of the product alkenes (Zaitsev's rule).
- However bulky bases such as t-butoxide (tBuO-) often lead to the anti-Zaitsev product.
- Similarly, subsituted cyclic halides may also give the anti-Zaitsev product.
- Stereoselectivity due to the preferred antiperiplanar (180o) arrangment of -H and -X in the transition state.